2015年ネパール春調査（0-2） Kathmandu Conference of International Glaciological Society

2015年ネパール春調査（0-2）

Kathmandu Conference of International Glaciological Society　（Mar.1～6, 2015）

Presentation; Why is a large glacial lake safe against GLOF?　

Presentation

Why is the large glacial lake of Nepal Himalaya safe against GLOF (Glacial Lake Outburst Flood)? This is a report of my field works in Himalaya since 1970’s.

01 Location of Khumbu, GLOF Lakes and Imja Glacial Lake and Gyajo Glacier.

   Since 1970, many glaciers are receding and glacial lakes are expanding in the Khumbu region of east Nepal, and the Mingbo GLOF　occurred in 1977. After that, the　Lagmoche  GLOF in 1985 and the Saboi GLOF in 1998 were occurred in the Khumbu region.
   The reported GLOFs occurred in the region are smaller glacial lakes which area are less than 1 km² and no larger glacial lakes such as Glaciers Imja and Tso Rolpa  showed any GLOF phenomena. Why is a large glacial lake safe against GLOF in Nepal Himalaya? 
   All of these valleys eroded by GLOF are clearly seen even in the satellite image due to the eroded valley topography. 　However, ther are no such eroded valley topography are seen in the river basins of  the larger glacial lakes, Imja and Tso Rolpa. 
   The man-made canal has already constructed at Tso Rolpa glacial lake in the Rolwaring region and is now under construction at Imja glacial lake in the Khumbu region.

02 1977  Mingbo GLOF

   The Mingbo glacial lake was located in the south of Mt. Ama Dablam, Khumbu region. On 3 Sept., 1977, the lake caused the GLOF when we had been making glaciological surveys in the region. So, we went up along the eroded Minbo valley caused by the GLOF (Left photo) and observed the vacant lake (Upper right) 300 m long, 200 m wide and 30 m deep. There was an ice-cored moraine (Left photo) and the terminal moraine was destroyed (Upper right). 
   Due to the large amount of debris caused by the GLOF, a new lake was formed in the Imja Khola near Pangboche (Lower left photo), while the river level of Dudh Kosi raised abruptly about 1 m at Raswa Hydrological Station (Lower right). So, the GLOF disasters, such as destruction of roads, bridges and houses near the river bed, were occurred along Dudh Kosi (river).

03 Lagmoche GLOF occurred  in 1985.

   The　Lagmoche (Digtso) glacial lake  near Thame in the western part of the Khumbu region has the steep cliff  causing avalanches and rock fall directly into the lake and huge waves (Tsunami) possibly destroyed  the end moraine (Upper right photo) and villages along the river (Lower right photo) by the 1985 GLOF.

04 1998 Saboi GLOF（Before and after Saboi GLOF）

   The Saboi glacial lake is in the south-eastern part of the Khumbu region was a small glacial lake in 1976 and has also the steep cliff in the upper part of the lake was thought to have destroyed the end moraine by a kind of Tsunami due to the avalanche or rock fall from the steep cliff at the time of the 1998 GLOF.

05 Madi GLOF

   The Gaptse glacial lake is located at the south face of Mt. AnnapurnaⅡ in the central Nepal Himalaya. Manoj and others (2005) reported that “the field verification was not done regarding the nature of glaciated region”  So, the field observation was carried out in May 2012 and we found that it is the lowest glacial lake in Nepal Himalaya with altitude of 2,500 m a.s.l.
   There is a quite large cliff, which altitude difference is about 4000 m, in the upper part of the glacial lake. We noticed frequent avalanches from Mt. AnnapurnaⅡand Lamjung Himal, and such avalanches and rock falls create a Tsunami to cause GLOF when they fall directly into the lake. The GLOF disaster occurred along the Madi Khola in 2003, 2005 and 2009 according to the local residents.

06 Study of Glacial Lake, Tulagi,  near Mt. Manaslu (compiled by GoogleEarth)

   The Tulagi glacial lake is one of the large glacial lake and located at the upper part of the Dana Khola, one of the Marshangdi’s tributary river in the west of Mt. Manaslu and Mt. Peak 29. Ther is no eroded valley topography caused by GLOF along Dana Khola below the river mouth of Tulagi glacial lake.

 07 Expansion of Glacial Lake, Tulagi near Mt. Manaslu

   The changes of the Tulagi glacial lake and its glacier terminus were determined by air photos and field surveys.The lake was 0.5 km wide, 3 km long in 2008 and  expanded at yearly rates of 31 m from 1975-1992, 47 m from 1992-2005, 68 m from 2005-2008, 60 m from 2008-2009, and no significant change from 2009-2014. The recent changing rate had been accelerating with active calving from 1975 to 2009, but it seems to be settled down and show no remarkable calving phenomena since 2009. The terminus of Tulagi glacier was thought to have stranded on the lake bed.

08 Tulagi glacier terminus taken on 25 Nov. 2008.

   The Tulagi glacial lake has the huge debris covered glacier in the upper part where the avalanches and rock falls occur, which is completely different from the small glacial lakes having the avalanches and rock falls directly dropping into the lake.

09 The above yellow arrows show the lowering of Tulagi lake level form 1996 to 2009.

   As Nepal Department of Hydrology and Meteorology (DHM) made a glaciological survey in 1996, the topographic characteristics were compared with the photographs of 1996 and 2009 in the lower part of the lake, and the lowering of the lake level is clearly shown by the yellow arrows and a clean pond is newly formed due to the sedimentation of glacial clay (glacier milk) at P point.

10 The above yellow arrows show the lowering of Tulagi lake level by 2.5 m  from 1996 to 2009.

   The shore line without vegetation is 2.5 m above the present lake level (Lower and upper right photo) and the 1996 water gauge has been left higher than the present lake level (Upper left photo). The lake level lowered 2.5 m from 1996 to 2009.

11 Thulagi field survey.

　　Thulagi field survey was carried out along the lake shore and moraines. 

12 The lowering of Tulagi lake level continues form 2005 to 2011.

   The lowering of the lake level was also found by checking both the GPS trail and Google Image, the 2009 GPS trail runs within the lake parallel to the lake shore of the 2005 Google Image (Upper right photo). However, the 2009 trail coincides with the shore line of the 2011 Google Image (Lower left photo) and this indicates the lowering of the lake level continued from 2005 to 2011.

13 Tsunami observed (2009.11.07)

   There are no avalanches and rock slides directly falling into the Tulagi glacial lake to create a Tsunami, but a small wave with 30 cm height occurred when the glacier terminus collapsed as calving phenomena and it will be one of agents to cause an erosion at the river mouth (outlet) between the glacial lake and the upper part of the down-stream river.

14 The lower part of Thulagi glacial lake

   The lower part of Thulagi glacial lake surrounded by huge moraine dammed up the Thulagi glacial lake. What is the age of this huge moraine?

15 Glacial history (glaciations) in Khumbu region (The 16^th century moraine)

   The plant remnants were sampled from the moraine basement of the Thuklha stage in the Khumbu region. Since the Carbon 14 age of the sample is 410±110 yr B.P., the age of the Thuklha stage is younger than 16th century.

16  Topography of the terminus of Tulagi glacier lake (Thulka Stage Moraine)

   Fushimi (1981) reported that the Tulagi glacial lake was formed after the glacial advance in 16th century and there is no evidence of the GLOF occurrence indicated by a newly formed river terrace with an eroded valley topography along Dana Khola. At the  end-moraine of Tulagi glacial lake between the lower part of the lake and the most upper part of Dana Khola, the river mouth (outlet) is eroded about 30 m from the top of the 16th century end moraine. So, the water level of Tulagi glacial lake has been lowered at the average annual rate of 5 cm due to the outlet erosion at the end moraine since the 16th century glacial advance that indicates to decrease the GLOF risk.

17 Tulagi Glacial Lake is safe against GLOF (Thulka Stage Moraine)

   The end moraine structure of Tulagi glacial lakes is wide and strong enough to prevent the occurrence of the GLOF. The lake level of the Tulagi glacial lake has been continuously lowered by the outlet erosion at the end moraine to decrease the GLOF risk and it is occurred without having a man-made canal. The Tulagi glacial lake is thought to have a kind of an autonomous property to prevent the GLOF.

18 Changes of the Imja glacial lake from 1975 to 2013. (Thulka Stage Moraine)

    The end moraine structure of Imja glacial lake is also wide and strong enough to prevent the occurrence of the GLOF

   This shows the Imja glacial lake taken in 1975 by plane, and in 2002 (Upper left photo) and 2013 (Lower left photo) on the field. The size of  Imja glacial lake expanded from 1975 to 2002, but reduced in 2013.

   Though the Himalayan Times (2013) reported that the United Nations Development Programme (UNDP) and Nepal government made an agreement to build a man-made canal, I would like to recommend to make necessary field observations before taking such project to construct a man-made canal in the pristine nature of the Himalayas.

19 Artificial canal of Tso Rrolpa glacial Lake (Thulka Stage Moraine)

   Though the man-made canal has already constructed at Tso Rolpa glacial lake in the Rolwaring region (Tama Khosi), I would like to make it sure whether it is appropriate or not to have a man-made canal at such a large glacial lake in Nepal Himalaya.

20  Conclusion; GLOF Map in Khumbu region, Nepal Himalaya.

   All of the reported GLOFs occurred in Nepal Himalaya are smaller glacial lakes and no larger glacial lakes such as Glaciers Tulagi, Imja and Tso Rolpa showed any GLOF phenomena. 
    Why is a large glacial lake safe against GLOF in Nepal Himalaya? What is the higher risk of the GLOF? It is not a large glacial lake, but a small glacial lake. We must be very careful about the small glacial lakes, for example, developing in the Hong Khola around Mt. Chamlang which have steep cliff to cause avalanches and rock falls that create a Tsunami destroying the end moraine and forming the GLOF, so they must be taken to mitigate against GLOF.

    Due to the outlet erosion at the end moraine of the larger glacial lake since the 16th century glacial advance, the lake level has been always lowering and reducing the GLOF risk without having a man-made canal. 

    As the ICIMOD (2011) reported “Imja glacial lake has less likelihood of outburst than Tulagi lake” and Tulagi glacial lake is safe against GLOF due to an erosion at the end moraine, so both the large glacial lakes such as Tulagi and Imja are safe against GLOF.